1. Field of the Invention
The present invention relates to a four fiber ring network optical switching circuit to be used at each node of an optical ring network system for carrying out a working/protection switching at optical level in a bidirectional optical ring network formed by four fibers, for example, and more specifically, to a four fiber ring network optical switching circuit for realizing a span switch function and a ring switch function by cross-connecting first and second working and protection input and output ports and first and second working and protection add/drop input and output ports for inputting and outputting working and protection signals transmitted through a ring network.
2. Description of the Background Art
As shown in FIGS. 4A and 4B, the four fiber ring optical network is formed by connecting a plurality of nodes A, B, C, D, E and F by a ring shaped transmission path comprising four optical fibers including a working fiber pair indicated by bold and thin solid lines and a protection fiber pair indicated by dashed lines. Here each node is referred to as a four fiber ring node. Also, one of the two fibers constituting each fiber pair will be used for an upward transmission while the other one will be used for a downward transmission.
In the four fiber ring optical network of this configuration, under the normal state, signals will be transmitted between arbitrary nodes through a route using the working fiber pair indicated by a bold solid line as a normal route in FIGS. 4A and 4B. In the example shown in FIGS. 4A and 4B, signals will be transmitted between nodes F and C by the normal route using the working fiber pair via nodes A and B.
When signals are transmitted between nodes F and C via nodes A and B by the normal route using the working fiber pair transmission path in this manner, if a trouble such as a span fault occurs in the working fiber pair transmission path of the normal route between nodes B and C as shown in FIG. 4A, for example, the span switching from the working fiber pair transmission path to a protection fiber pair transmission path is carried out by span switches at nodes B and C such that a transmission path connecting between nodes B and C is switched to the protection fiber pair transmission path indicated by a dashed line. As a result, the signal transmission between nodes F and C will be carried out by using the working fiber pair transmission path of the normal route between nodes A and F and between nodes A and B, and the protection fiber pair transmission path between nodes B and C.
Also, similarly, when signals are transmitted between nodes F and C via nodes A and B by the normal route using the working fiber pair transmission path, if a trouble such as a cable disconnection occurs in a fiber cable between nodes B and C such that both the working and protection fiber pair transmission paths are disconnected as shown in FIG. 4B, for example, the ring switching from the working fiber pair transmission path to the protection fiber pair transmission path is carried out by ring switches at nodes F, E, D and C such that the signal transmission between nodes F and C will be carried out by switching a transmission path to a bypass route via nodes E and D which is indicated by a bold solid line and using the protection fiber pair transmission path, so as to rescue the working signals.
A switch of the node for carrying out the span switching and the ring switching as described above is realized by the four fiber ring network optical switching circuit as shown in FIG. 5. This ring network optical switching circuit has first and second optical input/output ports (1) and (2) to which the protection and working fiber pairs in the first direction are connected, third and fourth optical input/output ports (3) and (4) to which the protection and working fiber pairs in the second direction are connected, fifth and sixth optical input/output ports (5) and (6) to be used as protection and working add/drop ports for transmission from the second direction to the first direction, and seventh and eighth optical input/output ports (7) and (8) to be used as working and protection add/drop ports for transmission from the first direction to the second direction.
In the ring network optical switching circuit of FIG. 5, the first optical input/output port (1) is directly connected with the fifth optical input/output port (5) and can be also connected with the sixth optical input/output port (6) via a span switch and with the seventh optical input/output port (7) via a ring switch. Also, the second optical input/output port (2) is directly connected with the sixth optical input/output port (6) and can be also connected with the first optical input/output port (1) via a span switch and with the third optical input/output port (3) and the eighth optical input/output port (8) via a ring switch. Also, the third optical input/output port (3) is directly connected with the eighth optical input/output port (8) and can be also connected with the seventh optical input/output port (7) via a span switch and with the sixth optical input/output port (6) via a ring switch. Also, the fourth optical input/output port (4) is directly connected with the seventh optical input/output port (7) and can be also connected with the third optical input/output port (3) via a span switch and with the first optical input/output port (1) and the fifth optical input/output port (5) via a ring switch.
Conventionally, the ring network optical switching circuit as shown in FIG. 5 is formed by using twelve 2xc3x972 optical switches as shown in FIG. 6.
In FIG. 6, SW-1, 3, 6 and 8 are ring switching 2xc3x972 optical switches, SW-2, 4, 5 and 7 are pass through 2xc3x972 optical switches, and SW-9, 10, 11 and 12 are span switching 2xc3x972 optical switches. Also, CP-1 is an optical coupler (optical branching element) for bridging with respect to a span switching to the first direction, CP-2 is an optical coupler (optical branching element) for bridging with respect to a span switching to the second direction, CP-3 is an optical coupler for bridging to the working fiber in the second direction with respect to a ring switching to the first direction, and CP-4 is an optical coupler for bridging to the working fiber in the first direction with respect to a ring switching to the second direction.
When the working signals are switched to the protection fiber pair transmission path by the span switching and the ring switching at a time of the trouble occurrence as in the cases shown in FIGS. 4A and 4B by using the ring network optical switching circuit as shown in FIG. 5 and FIG. 6, in order to automatically switching the working signals back to the working fiber pair at a time of the subsequent recovery from the trouble, there is a need for the so called bridge function for starting the transmission by switching the working signals into the protection fiber pair transmission path while continuing to transmit signals also to the working fiber pair transmission path on which the trouble has occurred, so that the conventional ring network optical switching circuit shown in FIG. 6 is equipped with a bridge function at times of the span switching and the ring switching as described above.
The conventional four fiber ring network optical switching circuit described above can be formed by a relatively small number of twelve 2xc3x972 optical switches, but if the optical switch is individually implemented, the optical switch itself becomes large and an entire structure of the ring network optical switching circuit also becomes large due to arrangements of optical fibers to be connected to the optical switches, and in conjunction with that there has been a problem that a cost required for the wiring processing becomes high.
It is therefore an object of the present invention to provide a four fiber ring network optical switching circuit capable of realizing the bridge function at times of the span switching and the ring switching economically by a very compact structure.
According to one aspect of the present invention there is provided a four fiber ring network optical switching circuit, comprising: a 10xc3x978 optical matrix switch having ten input ports and eight output ports; and two branching elements adapted to branch each one of two optical signals among eight optical signals that are inputs of the four fiber ring network optical switching circuit, into two identical optical signals, and to enter the two identical optical signals into two input ports of the 10xc3x978 optical matrix switch such that the eight optical signals are entered into the ten input ports of the 10xc3x978 optical matrix switch as ten optical signals.
According to another aspect of the present invention there is provided a four fiber ring network optical switching circuit to be used at nodes for connecting optical fiber transmission paths in a ring shape so as to form a bidirectional optical ring network, the four fiber ring network optical switching circuit comprising: first and second working and protection input and output ports and first and second working and protection add/drop input and output ports adapted to input and output working and protection signals to be transmitted through the bidirectional optical ring network; first and second branching elements adapted to branch each one of optical input signals for the first and second working add/drop input ports, and having first and second working branch-1 and branch-2 add/drop input ports for outputting branched optical signals; and an optical matrix switch adapted to cross-connect a plurality of input ports including the first and second working and protection input ports, the first and second protection add/drop input ports and the first and second branch-1 and branch-2 working add/drop input ports, and a plurality of output ports including the first and second working and protection output ports and the first and second working and protection add/drop output ports, so as to realize a span switching function and a ring switching function.
Other features and advantages of the present invention will become apparent from the following description taken in conjunction with the accompanying drawings.